A great number of methods for producing particles and/or filaments are described in the literature. Among these methods electrospray and electrospinning have an important highlight, because they produce particles and fibers, respectively.
Electrospray and electrospinning are technologies that use high electric fields for producing particles and/or fibers. In this process a jet of polymeric solution is accelerated and stretched through an electric field. Depending on the physical properties of the solution, the stretched jet can break, generating droplets, which produce micro/nanoparticles, or remain as a filament that after drying, produces fibers of micro/nanometric diameter (O. V. Salata, Tools of nanotechnology: Electrospray, Current Nanoscience 1: 25-33, 2005; S. Ramakrishna, et al., An Introduction to Electrospinning and Nanofibers, World Scientific Publishing Co., 2005).
Electrospray and electrospinning techniques make possible variations almost unlimited in the composition of ejected solutions, showing to be applicable in several technological sectors and for different applications, according to the needs of usage of particles or filaments.
Particles and filaments can be used in several industry segments, in the engineering of fabrics, ceramic fibers and filters, in the production of biomaterials used in treatment and diagnosis, in pharmaceutical, food, cosmetic industry etc. The particles and filaments also can be used in monitoring the pollutant dispersion, and in the quality of the environment protection processes.
The fundamental concepts of electrospray were launched by Lord Rayleigh, in 1882, when he was studying the instabilities in charged liquids (L. Rayleigh, On the equilibrium of liquid conducting masses charged with electricity, Phil. Mag. 14: 184, 1882). Applications of the technique were patented by J. F. Cooley e W. J. Morton (J. F. Cooley, Apparatus for Electrically Dispersing Fluids, U.S. Pat. No. 692,631, 1902; W. J. Morton, Method of Dispersing Fluids, U.S. Pat. No. 705,691, 1902). The explanation of the phenomenon was provided later by J. Zeleny (J. Zeleny, The electrical discharge from liquid points, and a hydrostatic method of measuring the electric intensity at their surfaces, Phys. Rev. 3:69-91, 1914) in 1914, but the physical principles of capillary formation in charged liquids only were established in 1964 by Taylor (G. I. Taylor, Disintegration of water drops in an electric field, Proceedings of the Royal Society 280: 383-397, 1964).
Regarding the electrospinning, which follows the same physical principles of electrospray, the first patent that described the technique, was registered in 1934 by Formhals (A. Formhals, Process and apparatus for preparing artificial threads, U.S. Pat. No. 1,975,504, 1934), when he was developing an apparatus for producing filaments from the force of electrostatic repulsion among the surface charges. Despite the apparatus for electrospinning is extremely simple, its operating mechanism, similar to the electrospray, is very complicated.
When a high voltage (usually in the range from 1 to 30 kV) is applied, the polymeric solution drop, in the ejector nozzle, becomes highly electrified with the charge uniformly distributed over the surface. As a result, the polymeric solution drop will suffer two types of Coulomb electrostatic forces, the repulsion among the surface charges and the force exerted by the external electric field. Under the action of these electrostatic interactions, the solution drop is distorted to a conic form, known as Taylor cone. Since the force of the electric field has exceeded a threshold value, the electrostatic forces can overcome the surface tension of the polymer solution, and then force the ejection of the solution jet from the ejector nozzle.
During the pathway that the electrified jet goes through, from the ejector nozzle to the collector, the process of stretching and lengthening of the jet takes place, and depending on the physical characteristics of the polymeric solution, the jet can break into drops or remain as a filament. In this pathway the evaporation of the solvent and the polymer solidification also take place, leading to the formation of particles or filaments (O. V. Salata, Tools of nanotechnology: Electrospray, Current Nanoscience 1: 25-33, 2005; S. Ramakrishna, et al., An Introduction to Electrospinning and Nanofibers, World Scientific Publishing Co., 2005).
Practically, all the polymers are susceptible to deposition by electrospray or electrospinning. The limitation is to find a solvent able to dilute or emulsify it in order to produce a solution or emulsion able to pass through the capillary of the pumping system. There are polymers for which there is some difficulty for deposition as a function of their physical or electrical properties, but adjusts of these parameters by means of the use of additives, variation of concentration etc, allow the use of these polymers.
Several polymers have been used industrially, such as Nylon, Polyester, Polyacrylonitrile, polyvynil alcohol, Polyurethane, Polylactic acid etc. Conventionally, the electrospinning technique uses preponderantly a solution of polymers in organic solvents, such as chloroform, formic acid, tetrahydrofuran (THF), dimethylformamide (DMF), acetone and alcoholic solvents.
The need for chemical activation of polymeric surfaces emerged together with the development of the first polymers. Generally, the simpler the polymeric chain, the smaller the reactivity. This generally implies in technical difficulties related mainly to dissolution and adhesion to other materials. The change of the polymers structure by introduction of new radicals in the chains, allowed generating new families of polymers with their own physicochemical properties.
In certain situations, it is necessary to use a polymer with an inert inside, but with reactive external surface in order to allow adhesion to other materials, or even to perform specific chemical reactions. Based on this need, from the beginning of the nineties, several techniques based on physical or chemical phenomena were developed, searching the superficial activation of polymeric materials. Among the several physical techniques employed, it is highlighted the electrostatic discharges at atmospheric pressure, the low energy ion implantation, and the low temperature plasma discharge in a reduced pressure environment.
The electrostatic discharges at atmospheric pressure consist in ionizing the environment air, or a gas at atmospheric pressure nearby the surface of an inert polymeric material. Such a phenomenon promotes chemical reactions between the reactive species generated by discharge and the polymer surface. Their main advantages are the simplicity and low cost of technique execution; however, their great disadvantage is the susceptibility of the material activated when exposed to the environment, reacting with any compounds present in the atmosphere and returning to make passive the surface or, even contaminating it (R. A. Wolf, Surface activation systems for optimizing adhesion to polymers, ANTEC™ 2004, Conference Proceedings).
The low energy ion implantation technique consists in producing and accelerating ions of interest, against the polymeric surface with controlled energy. This technique is extremely sophisticated and expensive, but allows to select the ions and to control their energies. Furthermore, the technique uses an ion beam extremely collimated, reaching a reduced area to be activated, what makes the processing of great areas, difficult and slow (G. Mesyats et al., Adhesion of polytetrafluorethylene modified by an ion beam, Vacuum 52:285-289, 1999).
The third technique consists in the exposure of polymeric surface to a low temperature plasma discharge, in a reduced pressure environment. The discharge in plasma allows a reasonable control of existing active species as a function of the gases employed to generate plasma. Depending on the plasma characteristics, this technique also can be known as plasma-immersion ion implantation (A. Kondyurin et al., Attachment of horseradish peroxidase to polytetrafluorethylene (teflon) after plasma immersion ion implantation, Acta Biomaterialia 4:1218-1225, 2008). The control of pressure and reaction gases flow allows controlling the concentration of active species and, consequently, the final activation degree of the produced surface. The need for vacuum in the environment before injection of reactive gases raises the costs and makes difficult the process (P. K. Chu et al., Plasma-surface modification of biomaterials, Mater. Sci. Eng. R36:143-206, 2002).
Among the great variety of chemical techniques for superficial activation, it is highlighted those of copolymer synthesis combining polymers chemically inert and active directly. These techniques have several economic advantages as easy manufacturing inclusive for the staggering process. However, from the distinct characteristics of surface energy of the employed polymers, the active sites can migrate to the inside of the inert polymer, reducing or eliminating totally the final product activity. Such phenomenon is highlighted as a disadvantage of the process. An option for eliminating this problem consists in grafting a layer of active polymer on an inert polymer substrate. In some cases, the grafting is aided by plasma. Even solving the problem of migration of the active sites, this process implies in the increase of steps necessary for obtaining the final product, (K. Kato et al., Polymer surface with graft chains, Progress in Polymer Science 28:209-259, 2003).
Other techniques for modifying the polymeric surfaces involve also treatments with solvents, acid or basic solutions and mechanical abrasion V. I. Kestelman, Physical methods of polymer material modification, Khimiya (Moscow), 1980). Most of these techniques present certain disadvantages, as for example, the production of industrial effluents, excessive degradation of the polymer, high production costs, aggregation of undesirable aspects to the polymer properties etc.